Inlets for propulsion systems for high speed supersonic aircraft are designed to efficiently decelerate the approaching high-speed airflow to velocities that are compatible with efficient airbreathing engine operation and to provide optimum matching of inlet airflow supply to engine airflow requirements. Entrance airflow velocities to existing air-breathing engines must be subsonic; therefore, it is necessary to decelerate the airflow speed during supersonic flight. The airflow velocities are slowed from supersonic speeds (above the speed of sound) to engine entrance Mach numbers that are subsonic (below the speed of sound).
In aircraft propulsion systems having supersonic inlets, it is essential that the inlet decelerate the airflow in a manner that minimizes the pressure losses, cowl and additive drag, and flow distortion at the engine entrance. For supersonic inlets, efficient deceleration of the supersonic velocities is accomplished by a series of weak shock waves and/or isentropic compression, in which the speed is progressively slowed to an inlet throat Mach number of about 1.30. A terminal shock wave located near the inlet throat slows the airflow from supersonic speeds (above the speed of sound) to subsonic speeds (below the speed of sound). This terminal shock wave typically changes a Mach 1.3 flow condition to a high subsonic flow level. Downstream of the terminal shock, the speed of the airflow is additionally slowed in the subsonic diffuser of the inlet by a smooth transitioning of the flow duct from a smaller throat area to the larger area at the engine entrance.
Mixed-compression inlets, in which some of the supersonic compression or deceleration in velocity is accomplished external to the duct and some of the compression is accomplished internally, have commonly been proposed for supersonic aircraft that cruise at Mach numbers higher than 2.0. Any inlet that accomplishes some of its compression internally is subject to an undesirable phenomenon known as inlet unstart. Inlet unstart is characterized by an expulsion of the inlet terminal shock from the desirable location at the inlet throat station to a position ahead of the inlet cowling with an associated large increase in drag and large thrust loss. Unstart may also affect the aerodynamics of the aircraft.
Sonic boom is another factor that must be taken into account in the design of inlets of supersonic aircraft. Since economically viable supersonic commercial aircraft must be able to operate supersonically over land, the inlet should contribute minimally to the sonic boom signature of the aircraft. Therefore, the technical challenge for the designer of inlets for modern commercial aircraft is to provide a high performance configuration that provides large operability margins (terminal shock stability to reduce the probability of inlet unstart), and to also identify a design that offers a reduction in the overall sonic boom signature of the aircraft. Mixed-compression inlets can efficiently decelerate the airflow while providing large operability margins. However, the external compression, which is provided by a centerbody or cowl surface, radiates shock waves outward that contribute to the aircraft's sonic boom signature. These designs also have leading edges that include an external surface at an angle to the local airflow. Oblique shock waves are generated by these surfaces, contributing to the aircraft's overall sonic boom problem. Over-land operation of commercial supersonic aircraft requires that the sonic boom signature from the aircraft be reduced to acceptable levels. In order to achieve the required acceptable boom levels, sonic boom contributions from each component on the aircraft must be reduced to the lowest possible level.
All-internal compression inlets are desirable from a sonic boom reduction standpoint, because they may be designed with no oblique shock waves generated by an external compression system that would contribute to sonic boom signature. However, attempts to design these inlets have been generally unsuccessful, primarily due to large amounts of bleed required for inlet starting and started operation. Since these designs typically utilized fixed geometry, large amounts of bleed were necessary to provide the effective flow area ratio from the inlet entrance to inlet throat to allow the inlet to start (establish a supersonic flow field from the inlet entrance to the inlet throat). Large amounts of bleed were also necessary during normal operation because these inlets did not incorporated a stability system. This trend is typical of inlets that do not incorporate a stability system. Adequate inlet stability margins for inlet operation prior to unstart can only be provided by the fixed geometry bleed systems by prohibitively bleeding large amounts of bleed airflow during normal operation. The development of a low sonic boom aircraft therefore requires an innovation in supersonic inlet design.